1. Field of the Invention
The present invention relates to a tracking type laser interferometer and a method for resetting the same, and in particular, to a tracking type laser interferometer and a method for resetting the same, which are preferable for a tracking type laser interferometer that detects displacement of a retroreflector being an object to be measured by utilizing interference of a laser beam, which is irradiated onto the retroreflector and is reflected by the retroreflector in the returning direction, and carries out tracking by means of a two-axis turning mechanism using a change in the position of the optical axis of the laser beam, and are capable of automatically resetting the laser interferometer to a tracking state even if the tracking is made impossible due to interruption of the laser beam, etc., or are capable of automating initial adjustment work when measurement is commenced.
2. Description of the Related Art
Tracking type laser interferometers that were disclosed by Japanese Published Unexamined Patent Application No. 2007-057522 (Patent Document 1), Japanese Patent No. 2603429 (Patent Document 2), and U.S. Pat. No. 6,147,748 (Patent Document 3) are available as a tracking type laser interferometer that measures the displacement and position of a moving body with high accuracy while tracking the moving body. Representatively, a description is given of a case where a homodyne Michelson type laser interferometer is employed that is described in Patent Document 1, using FIG. 1 and FIG. 2.
FIG. 1 is a perspective view showing the entire configuration of a conventional tracking type laser interferometer, and FIG. 2 is a brief configurational view showing the portion of an interferometer, respectively.
As shown in FIG. 1, a tracking type laser interferometer according to Patent Document 1 includes a light source portion 100, a main body portion 200, a retroreflector 300, a circuit portion 400 and a personal computer (PC) 500.
The light source portion 100 includes a frequency stabilized He—Ne laser light source 110, a lens 120 and an optical fiber 130.
The main body portion 200 includes a measurement portion 220, a two-axis turning mechanism 240, a reference sphere 260, and a base 280. The two-axis turning mechanism 240 is fixed on the base 280, and the measurement portion 220 is fixed on the two-axis turning mechanism 240. And, the reference sphere 260 is fixed on the base 280, and the center of the reference sphere 260 is made coincident with the center of turning of the two-axis turning mechanism 240.
Herein, the measurement portion 220 includes a displacement gauge 221 and an interferometer 230. And, the interferometer 230 includes, as shown in FIG. 2, a collimator lens 231, a polarization beam splitter (PBS) 232, λ/4 plates 233, 236, a planar mirror 234, a non-polarization beam splitter (NPBS) 235, a polarization plate 237, a two-dimensional PSD (Position Sensing Detector) or a quadruplicate divisional photo diode (QPD) 238, and a detector 239. The two-axis turning mechanism 240 includes a carriage 242, an elevation angle motor 244, and an azimuth angle motor 246.
The circuit portion 400 includes signal processing circuits 410 through 430 and motor drive circuits 440 and 450.
Hereinafter, a description is given of the actions thereof, using FIG. 1 and FIG. 2.
A laser beam incident into the interferometer 230 is emitted from the frequency stabilized He—Ne laser light source 110, and is made incident into the interferometer 230 through the lens 120 and the optical fiber 130. A laser beam incident into the interferometer 230 is split into two by the PBS 232, one of which is used as reference light for measurement of length, and the other of which is emitted to the retroreflector 300. The laser beam 102 emitted to the retroreflector 300 is again made incident into the interferometer 230 having the λ/4 plate 236 after being reflected by the retroreflector 300. The laser beam 102 that is again made incident into the interferometer 230 is split into two by means of the NPBS 235, one of which interferes with the reference light as measurement light. The detector 239 detects a change in intensity of the interference light, and the change in intensity is processed by the signal processing circuit 410, wherein displacement ΔL1 between the retroreflector 300 and the interferometer 230 is measured by using the PC 500. The displacement gauge 221 is an electrostatic capacitance type displacement gauge or an eddy current type displacement gauge, which detects displacement with respect to the reference sphere 260, and the displacement is processed by the signal processing circuit 420, wherein the displacement ΔL2 between the reference sphere 260 and the displacement gauge 221 is measured by using the PC 500. By adding ΔL1 to ΔL2 on the PC 500, it is possible to obtain displacement ΔL between the retroreflector 300 and the reference sphere 260.
On the other hand, another laser beam 102 which is split into two by the NPBS 235 is made incident into the QPD 238 that is capable of detecting the distance (hereinafter referred to as a “tracking error amount ΔTr) between the optical axis of the laser beam 102 incident into the retroreflector 300 and the center position of the retroreflector 300. Herein, the QPD 238 can measure the ΔTr in terms of dividing the component in two directions orthogonal to each other. For example, as shown in FIG. 1, where it is assumed that the optical axis of the laser beam 102 emitted from the interferometer 230 is a Z axis, the axis in the horizontal direction, which is perpendicular to the Z axis, is an X axis, and the axis orthogonal to the Z axis and X axis is a Y axis, the QPD 238 can detect the X-axis direction component ΔTrX and the Y-axis direction component ΔTrY with respect to ΔTr. Therefore, signals responsive to the ΔTrX and ΔTrY are brought into the PC 500 via the signal processing circuit 430, and control signals responsive to the values of ΔTrX and ΔTrY are given to the motor drive circuits 440 and 450. In response to the control signals thus given, the motor drive circuits 440 and 450 drive the elevation angle motor 244 and the azimuth angle motor 246 and rotates the carriage 242 in the elevation angle direction and the azimuth angle direction, wherein the center position of the retroreflector 300 and the optical axis of the laser emitted from the interferometer 230 are controlled so as to become coincident with each other in order to carry out tracking.
However, there may be cases where a conventional homodyne tracking type laser interferometer that is represented by the invention disclosed in Patent Document 1 described above becomes unable to track the position of the retroreflector 300 when the laser light is interrupted due to existence of an obstacle between the interferometer 230 and the retroreflector 300 and when the laser light is interrupted due to other reasons. In these cases, it is necessary for an operator to go to the tracking type laser interferometer and to adjust either one of the position of the retroreflector 300 or the emission direction of the laser emitted from the laser interferometer 230 so that the laser beam 102 can be irradiated from the position of the main body portion 200 having the interferometer 230 onto the retroreflector 300 and the QPD 238 existing in the interferometer 230 can detect the reflection light from the retroreflector 300. In addition, since the work is manually carried out, a change in temperature and generation of vibrations and foreign substances due to movement of a human being at this time become factors to change the measurement environment, wherein it adversely influences highly accurate measurement. And, since the measurement is also manually commenced by executing initial adjustment work after the tracking type laser interferometer is installed, a similar problem exists.
And, such a situation is not unique to Patent Document 1, wherein it is a common problem to Patent Documents 2 and 3.